# Azimuthally differential pion femtoscopy relative to the second and third harmonic in Pb-Pb 2.76 TeV collision from ALICE

## TL;DR

This paper investigates azimuthally differential pion femtoscopy in Pb-Pb collisions at 2.76 TeV, revealing how source geometry and flow fields influence femtoscopic measurements and their relation to initial collision conditions.

## Contribution

It introduces a detailed analysis of femtoscopic source shape deformation relative to second and third harmonic event planes using event shape engineering techniques.

## Key findings

- Radii modulations reflect source geometry and flow velocity fields.
- Event shape selection enhances understanding of initial geometry effects.
- Results are compared with theoretical models.

## Abstract

Azimuthally differential femtoscopic measurements, being sensitive to spatio-temporal characteristics of the source as well as to the collective velocity fields at freeze-out, provide very important information on the nature and dynamics of the system evolution. While the HBT radii modulations relative to the second harmonic event plane reflect mostly the spatial geometry of the source, the third harmonic results are mostly defined by the velocity fields. Radii variations with respect to the third harmonic event plane unambiguously signal a collective expansion and anisotropy in the flow fields. Event shape engineering (ESE) is a technique proposed to select events corresponding to a particular shape. Azimuthally differential HBT combined with ESE allows for a detailed analysis of the relation between initial geometry, anisotropic flow and the deformation of source shape. We present azimuthally differential pion femtoscopy with respect to second and third harmonic event planes as a function of the pion transverse momentum for different collision centralities in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV. All these results are compared to existing models. The effects of the selection of the events with high elliptic or triangular flow are also presented.